One of the most frequent causes of the theft of motor vehicles is the relative ease with which an ignition key can be acquired or reproduced. All too often, the driver of a vehicle leaves the ignition key inserted in the ignition lock. On the other hand, it is relatively easy for a thief to obtain a reproduction of a key by taking an impression or simply by reading the code number engraved on the key.
Various measures for reducing this risk have already been proposed, for example an acoustic warning signal which sounds when the driver has forgotten to withdraw the key, omission of the engraving of the code number on the key, etc. However, it has not been possible to eliminate the risk of theft completely by these measures.